Two stage turbochargers for internal combustion engines are known in the art and comprise a High Pressure (HP) turbocharger and a Low Pressure (LP) turbocharger, each turbocharger in turn comprising a compressor rotationally coupled to a turbine.
These turbocharger systems may be configured to operate both turbochargers at low/medium engine speeds and to operate the LP turbocharger only at high engine speed. In this second case, the HP turbocharger is bypassed.
Turbochargers allow higher internal combustion engine performances. However they are not free from drawbacks, in particular when the internal combustion engine is operated at low speed and/or low load.
A first drawback is, as an example, a not negligible thermal loss from the exhaust gases through the LP turbine housing, whose entity depends on the system's layout, and could be critical at low engine speed and load to enable the aftertreatment system to operate correctly.
The systems that treat the exhaust gases before they are emitted in the environment, in fact, need a minimum threshold temperature to guarantee a sufficiently high conversion efficiency, therefore as a result, aggressive warm-up strategies to compensate the temperature loss must be used. This results in a fuel consumption deterioration.
Within aftertreatment (AT) systems, the DPF (Diesel Particulate Filter), needs enough high and stable temperature during filter regeneration process from soot. Therefore the above mentioned thermal loss has to be recovered through injection of extra-fuel. This results, amongst others, in a reduced efficiency of soot oxidation requiring more frequent DPF regenerations and thus higher fuel consumption and oil dilution experienced by the customer. As additional drawback of more frequent regenerations, the AT is subjected to higher thermal stresses and ageing because of increased time exposure to high temperature reached during the regeneration process, with potential drawbacks on emission compliance.
Furthermore, the two stage turbocharger systems are affected, due to the turbine wheels and depending on the routing of the exhaust line from exhaust manifold to the aftertreatment system, by a higher exhaust back pressure (i.e. pressure after the exhaust valves) compared to single stage turbocharger systems. A similar impact is present on the intake side, where the low pressure stage adds a further pressure drop compared to a single stage compressor. As a result, being the exhaust manifold pressure usually higher than the inlet manifold pressure, there is a negative work done by the pistons (known as “pumping work”) that increases the fuel consumption.
In accordance with an embodiment disclosed is to provide an internal combustion engine with a two stage turbocharger system that is free from the aforementioned drawbacks.
In particular, the herein described embodiments of the present invention provide an internal combustion engine with high performance charging systems, ensuring high efficiency even at low operational speed.
Provided is an internal combustion engine having a two stage turbocharger and a method of operating thereof having features in accordance with the herein described embodiments.